How to handle users editing the amount they want to pay

I am currently working on a payment form. It's absolutely required that our users are allowed to edit the amount they want to pay.

Currently, the way I am handling this is to have an editable 'amount to pay' field. It will be prefilled with their current balance.

If a user edit this amount, what is the best way to warn them that they might be overpaying or underpaying their balance?

Main options I've considered:

• Do nothing, and let them continue. If they edited the amount, they are aware.
• Throw a modal that confirms they want to pay a different amount than balance owed
• Throw a toaster/javascript warning that they will probably see, and let them continue.

If users overpay:

• Their payment will be applied to this balance, other balances they might have, or refunded. We can't know straight away if they have other balances.

If users underpay:

• As long as they are before their due date, they are fine. If they underpay, then still have a balance by due date, they get a fee - same as if they just didn't pay.